The present invention relates to a distance measuring apparatus for measuring the distance to an object to be measured and, more particularly, to a distance measuring apparatus suitably used for the AF function of a camera.
Conventionally, Japanese Patent Publication No. 5-22843 has proposed skimming that uses a ring-shaped charge transfer portion to integrate signals and to also remove external light. FIGS. 26 to 29 are a schematic diagram, timing chart, and the like of an image sensing element (to be referred to as a skim CCD hereinafter) using skimming.
FIG. 26 is a schematic diagram illustrating the structure of the skim CCD, which, in this case, has three photoelectric conversion elements. In this skim CCD, a light-emitting element such as an IRED 1014 is caused to flicker in a pulse pattern to project light pulses onto the object to be measured. Light-receiving (photoelectric conversion) elements 1020 generate charges corresponding to the amounts of received light.
The skim CCD allows distance measurements in two modes, i.e., an active mode in which the reflected light is received and photoelectrically converted by the light-receiving elements 1020 and the photoelectrically converted charge signals are accumulated, and a passive mode in which the IRED 1014 does not emit light and charges corresponding to only external light are accumulated. The skim CCD is a hybrid type distance measuring apparatus which starts a passive distance measurement operation when a reliable distance measurement result cannot be obtained in an active distance measurement operation.
Each light-receiving sensor (element) 1020 corresponds to a pair of ON and OFF pixels 1022 and 1023, and a charge accumulated by each light-receiving sensor 1020 is transferred to the corresponding ON or OFF pixel 1022 or 1023. Thereafter, the charge is transferred to a ring 1021.
FIG. 27 is a timing chart showing the charge transfer timings. As shown in FIG. 27, the charge accumulated in each sensor 1020 during the High (ON) period of an accumulation time signal is transferred to the corresponding ON pixel 1022 in synchronism with a High ON pixel transfer signal.
On the other hand, the charge accumulated in each sensor 1020 during the Low (OFF) period of the accumulation time signal is transferred to the corresponding OFF pixel 1023 in synchronism with a High OFF pixel transfer signal.
After charges accumulated in pixels X, Y, and Z are transferred to the corresponding ON and OFF pixels 1022 and 1023, the pixel data are transferred to the ring 1021 in synchronism with a High ring transfer signal. The ring transfer signal is synchronously output so that ON pixels X are added to each other.
In the active distance measurements, an IRED terminal (not shown) is turned on/off in synchronism with the accumulation time signal. In the passive distance measurements, the IRED terminal is kept OFF irrespective of the accumulation time signal. Note that numerals 1, 2, and 3 in FIG. 27 indicate the numbers of rounds on the ring, and correspond to one, two, and three rounds on the ring shown in FIG. 28.
The charge accumulation amount per round can be adjusted by adjusting the accumulation time or the driving frequency of the skim CCD. Note that the accumulation time can be used for decreasing the charge accumulation amount of each ON pixel since the ratio of High/Low periods of the accumulation time signal can be determined by communications from, e.g., a microcomputer 2001 shown in FIG. 17 (to be described later) to a sensor control unit 2008.
Also, since the accumulation time signal is generated by frequency-dividing the driving frequency of the skim CCD, the accumulation time of each ON/OFF pixel per round can be shortened by increasing the driving frequency.
On the other hand, by decreasing the driving frequency, the accumulation time of each ON/OFF pixel per round can be prolonged, and consequently, the charge accumulation amount per round can be increased.
FIG. 28 shows the charge accumulation amount per round on the ring, and also shows the states wherein the respective charge accumulation amounts are added to each other as the number of rounds on the ring increases. The number of rounds on the ring indicates the number of transfers of pixel signals to the ring. One round on the ring represents that the accumulated charge is transferred only once, and accumulation is also performed only once. Three rounds on the ring represent that accumulation is performed three times, and the ring stores the sum total of accumulated charges obtained by the three accumulations.
When the signal charge obtained per round on the ring does not have a sufficient level, the number of times of accumulation is increased to sequentially add signal charges so as to obtain a signal charge with a high S/N ratio.
Table 1 below briefly summarizes the features and merits of the active and passive distance measurement methods.
TABLE 1 ______________________________________ Suited Suited Object Suited Object Distance Luminance ______________________________________ Active Object with from near from about high distance to EV3 to EV14 reflectance middle distance (several m) Passive Object with from several from about high m to far EV8 to EV18 contrast distance ______________________________________
As shown in Table 1 above, the active distance measurement method is good for a near, high-reflectance object, and distance measurements from a low-luminance, dark place to a relatively high-luminance place, and the passive distance measurement method is good for a far, high-contrast object and distance measurements in a bright place.
For this reason, in the prior art, in order to shorten the distance measurement time, the driving frequency of the skim CCD is changed between the active and passive distance measurement methods so as to ignore distance measurements in a dark place that often produces distance measurement results with low reliability.
FIG. 29 is a flow chart showing the conventional AF sequence.
As shown in FIG. 29, when the AF operation starts in the first step S501, the active distance measurement mode is set in step S502. In step S503, it is checked if the current mode is the active or passive mode. As a result of checking, if the current mode is the active mode, the initial value of the driving frequency (fc) is set to be 500 kHz in step S504; if the current mode is the passive mode, the initial value of the driving frequency (fc) is set to be 1 MHz in step S513.
In step S505, an ICG (Integration Clear Gate) mode is executed at the selected driving frequency and by driving to flicker a light projection unit such as an IRED if the active mode is selected. In the ICG mode, external light components incident on the skim CCD are measured to change the accumulation conditions such as the driving frequency, accumulation time, and the like, so as to accumulate charges under optimal conditions without saturating the skim CCD.
The ICG mode will be described later with reference to FIG. 30.
After the driving frequency, accumulation time, and the like are determined in the ICG mode, the maximum number of rounds on the ring for accumulation is set in step S506, and the flow then advances to step S507 to execute the integral mode.
The maximum number of rounds on the ring for accumulation is set to forcibly end the integral mode after charges are accumulated up to the maximum number of rounds on the ring for accumulation when the amount of light incident on the skim CCD is small and the obtained signal is not enough to perform distance measurement calculations (mainly, low-luminance, low-reflectance, far-distance objects, and the like).
When the integral mode has ended after charges are accumulated in sufficient amount or accumulation is repeated by the maximum number of rounds on the ring for accumulation, the flow advances to step S508 to execute the reading mode. In the reading mode, charges obtained in the integral mode are A/D-converted, and the converted data is stored in a memory of a microcomputer.
Thereafter, the object distance is calculated based on the obtained image data in step S509. After calculations, it is checked in step S510 if the current mode is the active or passive mode.
If the current mode is the active mode, the passive mode is set in step S514. More specifically, the light projection unit is turned off, and thereafter, the same operation as in the active mode is performed to calculate the object distance.
After the object distances are calculated using both the active and passive modes, as described above, the flow advances to step S511 to perform a distance measurement result selection calculation so as to select one of the active and passive distance measurement results. Thereafter, the AF operation ends in step S512.
The ICG mode will be explained below.
FIG. 30 is a flow chart showing the sequence executed in the conventional ICG mode.
When the ICG mode starts in the first step S601, communications are made with a skim CCD such as a skim CCD 2024 shown in FIG. 17 (to be described later). The charge accumulation time and other accumulation conditions of the skim CCD can be changed by communicating with the skim CCD, and communication data are set to obtain the longest accumulation time as the initial value of the accumulation time.
After the communications are complete, the residual charges in the skim CCD are cleared in step S603. In step S604, charge accumulation is started, and at the same time, a signal SKOS output from the skim CCD is monitored.
Since this signal SKOS is inverted when a charge transfer channel has reached saturation during charge accumulation in the ICG mode, whether or not the current charge accumulation conditions are proper can be discriminated by monitoring the time required until the signal SKOS is inverted.
During the accumulation, it is checked in step S605 if the signal SKOS is inverted. If the signal SKOS is inverted, the number of rounds (time period) on the ring required until the signal SKOS is inverted is checked in step S608. If the count value (time period) until the signal SKOS is inverted is equal to or larger than a predetermined value (assumed to be four rounds in the flow chart), the flow advances to step S607 to end the ICG mode.
On the other hand, if the count value is smaller than the predetermined value (four rounds) in step S608, the current accumulation time is checked in step S609. If the current accumulation time is not the shortest time, the accumulation time is shortened in step S610 to repeat the ICG mode again.
If the current accumulation time is shortest, the flow advances to step S607 to end the ICG mode.
If it is determined in step S605 that the signal SKOS is not inverted, the current number of rounds on the ring for accumulation is checked in step S606. If the current number of rounds on the ring for accumulation has not reached the maximum number of rounds on the ring for accumulation yet, accumulation is repeated; otherwise, the flow advances to step S607 to end the ICG mode and to start the next integral mode.
However, in the above-mentioned prior art, during the process of injecting signal charges generated by a plurality of photoelectric conversion elements from a signal charge injection unit into a charge transfer channel via a signal charge supply unit, some signal charges are omitted to lower the far-distance measurement performance of the distance measuring apparatus considerably.
In the active distance measurement operation which receives light and performs accumulation when light beam pulses projected toward the object to be measured are reflected by the object to be measured and return to the apparatus, and calculates the distance to the object to be measured using the accumulated signal charges, when a reliable distance measurement result cannot be obtained in an active distance measurement operation, the distance measurement mode is switched to a passive distance measurement operation. However, when the distance measurement mode is switched to a passive distance measurement operation in a dark situation in which the luminance of the object to be measured and its surrounding portion is very low, the distance measurement time is nonsensically prolonged.
In the active distance measurement method, the maximum possible current is often supplied to an IRED (light-emitting diode) serving as a light projection unit that broadens the distance measurement range in the far-distance direction. However, when the flickering period of the IRED is set to be too long, heat produced by the current supplied through the IRED intermittently exceeds an allowable value, and the IRED deteriorates gradually.
As is generally known, when the driving frequency of the skim CCD is relatively low, good performance can be obtained in respect of the transfer efficiency of accumulated charges and the dark current. In this case, when the amount of incident light is large (under a high-luminance condition or the like), the charge transfer channel is readily saturated.
Conversely, when the driving frequency is high, the charge transfer channel is hard to be saturated but has poor transfer efficiency. As a consequence, signal components readily deteriorate.
FIG. 31 illustrates the influences of the driving frequency of the skim CCD and the number of rounds on the ring for accumulation on the accumulated charge amount. As can be seen from FIG. 31, the charge amount accumulated at a low driving frequency via a small number of rounds on the ring is the same as that accumulated at a high driving frequency via a large number of rounds on the ring.
However, in the prior art, since the maximum number of rounds on the ring for accumulation is constant independently of the driving frequency, if the maximum number of rounds on the ring for accumulation is set with reference to a high frequency, an extra accumulation time is required, and an over-specification problem is posed.
On the other hand, if the maximum number of rounds on the ring for accumulation is set with reference to a low frequency, a sufficient image signal cannot be obtained when the frequency is high.
In view of these problems, in the prior art, when the amount of incident light is large (high reflectance, high luminance), the driving frequency of the skim CCD is set to be a fixed value to prevent accumulated charges from being saturated, and the accumulation time is varied (electronic shutter function) in correspondence with the amount of incident light.
However, when the driving frequency is fixed and the charge accumulation time is decreased in correspondence with increases in amount of incident light, saturation of accumulated charges cannot be avoided depending on the selected driving frequency when the amount of incident light is large, and distance measurement calculations cannot be made.
Also, when the amount of incident light is small, signal components high enough to perform distance measurement calculations cannot be obtained, or signal components obtained based on light emitted by the IRED are buried in external light components. Furthermore, depending on the transfer efficiency upon transferring charges from the sensors to the ring, signal components obtained based on light emitted by the IRED are buried, and the S/N ratio is impaired, thus disturbing the distance measurement calculations.
In general, when the skim CCD is driven at a low driving frequency, it can assure high transfer efficiency of accumulated charges and can become strong against dark currents. However, in such case, if the amount of incident light is large (under a high-luminance condition), the charge transfer channel is readily saturated. Conversely, when the skim CCD is driven at a high driving frequency, the transfer efficiency of the charge transfer channel is impaired, and signal components tend to deteriorate.
Furthermore, since the conventional apparatus does not take the performance or characteristics of the active and passive modes described in Table 1 above into consideration, and drives the skim CCD at a low driving frequency as in the active mode so as to measure the distance to a low-luminance or low-contrast object that the passive mode is not good at, the distance measurement time becomes redundant, and only an unreliable distance measurement result is obtained.